1. Field of the Invention
The present invention relates to a body-worn personal communications apparatus, for example a wrist-carried wireless telephone.
2. Description of the Related Art
Wireless telephones are generally available either with microphones and loudspeakers built into the main body of the phone, which is then hand-held in a conventional manner, or with headsets which enable the user to operate in a hands-free manner. Progress in miniaturization of electronic components has enabled the production of body-worn personal wireless telephones, which have the advantage of being more convenient to carry and operate than a separate hand-held telephone. The most common example of such a device is a combined wristwatch and wireless telephone, but wireless telephones may also be incorporated in other body-worn goods, for example pendants.
Other types of body-worn personal communications devices include those providing wireless connectivity. An example of such a device is a wristwatch communicator operating according to the Bluetooth specification, which is aimed at short range ad-hoc data and voice wireless communications in both stationary and mobile communication environments. Communication takes place in the unlicensed ISM band at around 2.45 GHz. A wristwatch communicator could, for example, function as a user interface to an existing mobile phone, with communication between phone and wristwatch using a Bluetooth link.
A particular problem in the design of wrist-carried communicators is to provide an antenna which is efficient (to maximise battery life), compact (to avoid increasing the bulk of the device) and robust. A great deal of work has been performed in this area. The same problem occurs with other body-worn personal communications devices.
In a wrist-carried communicator the most common arrangement is to incorporate a magnetic loop antenna into the wristwatch strap, although this approach has to overcome the problems caused by varying wrist sizes and the joins between the strap and watch case and between the two ends of the strap. An example of a strap loop antenna is disclosed in U.S. Pat. No. 5,450,091 in which the capacitance between the overlapping strap ends is varied when the wristwatch is worn by users having different wrist sizes, thereby compensating for the change in loop dimensions. Such a design is difficult to make mechanically robust and reliable particularly where the strap is joined to the case. Further, it cannot be used with alternative body-worn transceivers which do not have a strap or equivalent structure into which to incorporate the antenna.
To overcome problems of robustness, attempts have been made to incorporate antennas into the watch casing itself. For example, U.S. Pat. No. 5,737,699 discloses a magnetic loop antenna formed within a watch casing. However, the need to accommodate the watch mechanism and transceiver circuitry in the casing complicates the design of the loop antenna and the assembly of the wristwatch telephone.
A number of other antenna designs for mounting inside a watch casing have been proposed, but all suffer from significant problems. Conformal dipoles can be compact but, because of their conformal nature, receive the component of the electric field parallel to the body which is significantly reduced by the presence of the body. Patch antennas with dimensions suitable for use in a watch casing have an inherently low bandwidth, for example between 1 and 2% fractional bandwidth at 2.4 GHz. This is much less than the fractional bandwidths required for many applications, for example 4% for Bluetooth (around 2.45 GHz), 9% for GSM (Global System for Mobile communications, operating from 880 to 960 MHz), and 15% for UMTS (Universal Mobile Telecommunications System, operating from 1900 to 2200 MHz).